Renal Replacement Therapy (RRT) has evolved from the long, slow hemodialysis treatment regime of the 1960's to a diverse set of therapy options, the vast majority of which employ high permeability membrane devices and ultrafiltration control systems.
Biologic kidneys remove metabolic waste products, other toxins, and excess water. They also maintain electrolyte balance and produce several hormones for a human or other mammalian body. An artificial kidney, also called a hemodialyzer or dialyzer, and attendant equipment and supplies are designed to replace the blood-cleansing functions of the biologic kidney. At the center of artificial kidney design is a semipermeable filter membrane that allows passage of water, electrolytes, and solute toxins to be removed from the blood. The membrane retains in the blood, the blood cells, plasma proteins and other larger elements of the blood.
Over the last 15 years, the intended use of the RRT equipment the system has evolved into a subset of treatment alternatives that are tailored to individual patient needs. They include ultrafiltration, hemodialysis, hemofiltration, and hemodiafiltration, all of which are delivered in a renal care environment, as well as hemoconcentration, which is typically delivered in open heart surgery. Renal replacement therapies may be performed either intermittently or continuously, in the acute or chronic renal setting, depending on the individual patient's needs.
Ultrafiltration involves the removal of excess fluid from the patient's blood by employing a pressure gradient across a semipermeable membrane of a high permeability hemofilter or dialyzer. For example, removal of excess fluid occurs in hemoconcentration at the conclusion of cardiopulmonary bypass surgery. Hemodialysis involves the removal of toxins from the patient's blood by employing diffusive transport through the semipermeable membrane, and requires an electrolyte solution (dialysate) flowing on the opposite side of the membrane to create a concentration gradient. A goal of dialysis is the removal of waste, toxic substances, and/or excess water from the patients's blood. Dialysis patients require removal of excess water from their blood because they lack the ability to rid their bodies of fluid through the normal urinary function.
One of the potential risks to health associated with RRT is hypotension, which is an abnormal decrease in the patient's blood pressure. An abnormally high or uncontrolled ultrafiltration rate may result in hypovolemic shock, hypotension, or both. If too much water is removed from the patient's blood, such as might occur if the ultrafiltration rate is too high or uncontrolled, the patient could suffer hypotension and/or go into hypovolemic shock. Accordingly, RRT treatments must be controlled to prevent hypotension.
Alternatively, a patient may experience fluid overload in his blood, as a result of fluid infusion therapy or hyperalimentation therapy. Certain kinds of RRT machine failures may result in fluid gain rather than fluid loss. Specifically, inverse ultrafiltration may result in unintended weight gain of a patient and is potentially hazardous. Uncontrolled infusion of fluid by whatever mechanism into the patient could result in fluid overload, with the most serious acute complication being pulmonary edema. These risks are similar in all acute and chronic renal replacement therapies (ultrafiltration, hemodialysis, hemofiltration, hemodiafiltration, hemoconcentration). Monitoring patients to detect excessive fluid loss is needed to avoid hypotension.
Rapid reduction in plasma or blood volume due to excessive ultrafiltration of water from blood may cause a patient to exhibit one or more of the following symptoms: hypovolemia-hypotension, diaphoresis, cramps, nausea, or vomiting. During treatment, plasma volume in the patient's blood would theoretically remain constant if the plasma refilling rate equaled the UF (ultrafiltration) rate. However, refilling of the plasma is often not completed during a RRT session. The delay in refilling the plasma can lead to insufficient blood volume in a patient.
There appears to be a “critical” blood volume value below which patients begin to have problems associated with hypovolemia (abnormally decreased blood volume). Fluid replenishing rate is the rate at which the fluid (water and electrolytes) can be recruited from tissue into the blood stream across permeable walls of capillaries. This way blood volume is maintained relatively constant. Most of patients can recruit fluid at the rate of 500 to 1000 mL/hour. When patients are treated at a faster fluid removal rate, they begin to experience symptomatic hypotension.
Hypotension is the manifestation of hypovolemia or a severe fluid misbalance. Symptomatically, hypotension may be experienced by the patient first as light-headedness. To monitor patients for hypotension, non-invasive blood pressure monitors (NIBP) are commonly used during RRT. When detected early, hypotension resulting from the excessive loss of fluid is easily reversed by giving the patient intravenous fluids. Following administering fluids the RRT operator can adjust the ultrafiltration rate to make the RRT treatment less aggressive.
Ultrafiltration controllers were developed specifically to reduce the occurrence of hypotension in dialysis patients. Ultrafiltration controllers can be based on approximation from the known trans-membrane pressure (TMP), volume based or gravity based. Roller pumps and weight scales are used in the latter to meter fluids. Ultrafiltration controllers ensure the rate of fluid removal from a patient's blood is close to the fluid removal setting that was selected by the operator. However, these controllers do not always protect the patient from hypotension. For example, the operator may set the fluid removal rate too high. If the operator setting is higher than the patient's fluid replenishing rate, the operator should reduce the rate setting when the signs of hypotension manifest. If the excessive rate is not reduced, the patient may still suffer from hypotension, even while the controller operates properly.
Attempts were made during the last two decades to develop monitors that could be used for feedback control of dialysis machine parameters, such as dialysate concentration, temperature, and ultrafiltration rate and ultrafiltrate volume. Blood volume feedback signals have been proposed that are based on optical measurements of hematocrit, blood viscosity and blood conductivity. Real time control devices have been proposed that adjust the ultrafiltration rate to maintain the blood volume constant, and thereby balance the fluid removal and fluid recruitment rates. None of these proposed designs led to significant commercialization owing to the high cost of sensors, high noise to signal ratio or lack of economic incentive for manufacturers. In addition, many of these proposed systems required monitoring of patients by highly trained personnel.
Controllers that protect from hypotension are especially needed for patients suffering from fluid overload due to chronic Congestive Heart Failure (CHF). In CHF patients, fluid overload typically is not accompanied by renal failure. In these patients mechanical solute (toxins) removal is not required. Only fluid (plasma water) removal is needed. Ideal Renal Replacement Therapy (RRT) for these patients is Slow Continuous Ultrafiltration (SCUF) also known as “Ultrafiltration without Dialysis”.
SCUF must be controlled to avoid inducing hypotension in the patient. Due to their poor heart condition, CHF patients are especially vulnerable to hypotension from excessively fast fluid removal. The clinical treatment objective for these patients can be formulated as: fluid removal at the maximum rate obtainable without the risk of hypotension. This maximum rate is equivalent to fluid removal at the maximum rate at which the vascular volume can be refilled from tissue. This maximum rate for CHF patients is typically in the 100 to 1,000 mL/hour range. The rate can vary with the patient's condition and is almost impossible to predict. The rate can also change over the course of treatment, especially if the objective of treatment is to remove 2 to 10 liters of fluid.
Hypotension in CHF patients often results from a decrease of the cardiac output of the patient. Cardiac output is the volume of blood that is ejected per minute from the heart as a result of heart contractions. The heart pumps approximately 4–8 L/min in a normal person. In a CHF patient cardiac output most often decreases because the heart is subject to a reduction of filling pressure. This dependency on the filling pressure is a well-known clinical consequence of the deterioration of the heart muscle during CHF. In a healthy person when the heart filling pressure is lowed, the heart will compensate and maintain cardiac output by working (e.g. pumping) harder. Filling pressure is the blood pressure in the right atrium of the heart. This pressure is approximately equal to the patient's venous pressure measured elsewhere in a great or central vein (such as vena cava) and corrected for gravity. In a fluid overloaded CHF patient Central Venous Pressure (CVP) is typically between 10 and 20 mmHg. If this pressure drops by 5 to 10 mmHg, the patient is likely to become hypotensive soon.
The danger of hypotension as a consequence of excessive fluid removal during dialysis and other extracorporeal blood treatments has been recognized. U.S. Pat. No. 5,346,472 describes a control system to prevent hypotension that automatically adjusts the sodium concentration added to the dialysate by infusing a hypertonic or isotonic saline solution in response to operator input or patient's request based on symptoms. European patent EU 0311709 to Levin and Zasuwa describes automatic ultrafiltration feedback based on arterial blood pressure and heart rate. U.S. Pat. No. 4,710,164 describes an automatic ultrafiltration feedback device based on arterial blood pressure and heart rate. U.S. Pat. No. 4,466,804 describes an extracorporeal circulation system with a blood oxygenator that manipulates the withdrawal of blood to maintain CVP constant. U.S. Pat. No. 5,938,938 describes an automatic dialysis machine that controls ultrafiltration rate based on weight loss or the calculated blood volume change. Late model AK200 dialysis machines from Gambro (Sweden) include an optional blood volume monitor called BVS or Blood Volume Sensor. This sensor is optical and in fact measures blood hematocrit or the concentration of red blood cells in blood. Since dialysis filter membranes are impermeable to blood cells, increased hematocrit signifies the reduction of the overall blood volume. The BVS sensor is not included in a feedback to the machine and is used to help the operator assess the rate of fluid removal.
U.S. Pat. No. 5,346,472 describes a mixed venous oxygen saturation responsive system for treating a malfunctioning heart. By sensing the change of the oxygen content in the venous blood the system adjusts the operation of a heart pacemaker. However, venous saturation of blood has never been used in adjusting an extracorporeal blood therapy for fluid removal such as ultrafiltration, hemofiltration or dialysis.
Other devices have been proposed that use arterial pressure as a feedback to the ultrafiltration controller to avoid hypotension. Automatic Non-Invasive Blood Pressure (NIBP) monitor feedback was used as a control system input. NIBP measures systolic and diastolic arterial blood pressure by periodically inflating a blood pressure cuff around the patient's arm or leg. Acoustic or oscillatory methods detect the pressure level at which blood vessels collapse. This level approximates systemic arterial blood pressure. Closed loop dialysis or fluid removal devices designed around this principle have several inherent deficiencies, including:                a) NIBP is inaccurate. Errors of up to 20 mmHg can be expected in the system. To avoid system oscillations and false alarms, the feedback would have to be slow and heavily filtered.        b) NIBP is not continuous, but is rather based on periodic pressure measurements. If the blood pressure cuff were inflated more frequently, less than every 15 minutes a patient would experience significant discomfort. Also, blood vessels change their elasticity from the frequent compressions of the blood cuff. This change in elasticity can add to the inaccuracy of cuff pressure measurements.        c) The arterial pressure in a CHF patient does not drop immediately following the reduction of cardiac output. It may take considerable time for a CHF patient to exhaust their cardiac reserve. By that time, the hypotension would have already occurred and its reversal would require medical intervention. Accordingly, hypotension may occur before NIBP detects it.        d) In a CHF patient, arterial blood pressure is maintained by the body to protect the brain. Neurohormonal signals are sent in response to baroreceptors that cause vasoconstriction of blood vessels to legs, intestine and kidneys. By sacrificing other body organs needs, arterial blood pressure to the brain can be kept constant at the expense of reduced blood flow to organs while the cardiac output is reduced dramatically.        
Altogether, hypotension in a CHF patient can create a dangerous situation when the arterial blood pressure is apparently normal, while the overall condition of the patient is worsening. By the time the NIBP measurement has detected hypotension, serious medical intervention may be needed.
It is desired to have a feedback based control system that will continuously and automatically manipulate the ultrafiltration rate to achieve optimal ultrafiltration. In such a system, fluid is removed rapidly and without the risk of hypotension. It is also desired, in the application to CHF patients, to anticipate and correct the onset of the condition that before it is manifested by the reduction of arterial pressure.